Our aim is to identify DNA sequences in the fetal and adult globin gene promoters that participate in the developmental regulation of these genes. We constructed composite promoters containing portions of the fetal Gamma and adult Beta 5' flanking regions and assessed their function in stably transformed human K562 cells, a cell line that expresses the Gamma globin gene but not the Beta. A 270 base pair fragment from the Gamma flanking region activated the Beta globin promoter. The reversed sequence of this Gamma fragment did not activate the Beta promoter. A corresponding region of the human Zeta globin gene, which is also expressed by the K562 line, appeared to function in a similar way. Further dissection of the Gamma flanking region may delineate specific sequences involved in globin gene regulation in these cells.